Study sheds light on risks of being a second twin

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For twins born preterm (before 36 weeks), there was no difference in overall death rates between the first and second. This is because the background rate of death is already high for both, due to the effects of prematurity.

However, the researchers found that second twins born at or after the full term of pregnancy (36 weeks and over) were twice as likely to die during labour or shortly after birth. This excess risk appeared to be due to complications arising from the birth process.

Vaginal delivery of the second twin is widely accepted as a time of high risk, but previous research into the impact of birth order on twin morbidity has produced an array of inconsistent findings. A number of studies have suggested that second born twins have significantly elevated levels of mortality compared to their elder siblings, while other surveys have concluded that birth order has no discernable impact on either twin's chance of survival.

In order to clarify this important area, British researchers conducted an analysis of all twin pregnancies with one death during or shortly after the delivery stage in England, Northern Ireland and Wales between 1994 and 2003.

Data were obtained from a combination of official sources that have recorded twin deaths since the early 1990s, including the Confidential Enquiry into Maternal and Child Health (CEMACH), the Confidential Enquiry into Stillbirths and Deaths in Infancy (CESDI) and Office for National Statistics (ONS). To prevent any distortion of the analysis, each case was screened to remove instances where the fatality was caused by factors not related directly to the delivery process, including where the twin suffered from an inherent abnormality or when therapeutic abortions were performed.

In total, 1377 twin pregnancies were highlighted as meeting these strict criteria and all were analysed to identify the association between birth order and the risk of death. Additional factors were also analysed to assess whether they influenced the relationship between birth order and the risk of death, such as the length of gestation, different methods of delivery and the mother's age and ethnicity.

The results of this study showed that there was no association between birth order and the overall risk of death among preterm twins. The mother's age and ethnic group also had no discernable impact on twin mortality.

However, the analysis demonstrated that second twins born at term had a greater than twofold chance of death during or shortly after the delivery phase as a result of complications caused by the birth process. Moreover, the excess risk to the second twin at term was lower if a caesarean section was performed.

The authors caution though that further research needs to be conducted to prove conclusively that caesareans provide a protective effect for second born twins born at term.

BMJ-British Medical Journal